In a traditional system of starting a prime mover, such as a gas turbine engine, a starter motor is used to apply torque to the turbine engine's shaft for some period. As described in U.S. Pat. No. 6,035,626 issued to Wahl et al., the entire content of which being incorporated herein by reference, the gas turbine engine includes a compressor, a combustor, and a turbine. The compressor and the turbine are mounted for rotation on a shaft, which may also be used to drive other components, such as a gearbox and various accessories, such as an alternating current (AC) electric generator and lube pump.
As described in the Wahl Patent, as the shaft starts to rotate, air is inducted into the compressor, compressed and then discharged in the combustor. Concurrently, the engine's fuel control system feeds fuel into the combustor in accordance with a preprogrammed fuel schedule to precisely maintain the proper fuel to air ratio in the combustor. At a rotational speed of about 10 to 20 percent of the engine's operating speed, the conditions in the combustor become such that the fuel/air mixture can be ignited at a stage commonly known as “light-off”. Should the fuel to air ratio be either too rich or too lean, light-off will not occur and the engine will experience a “hung start”. After light-off the starter motor torque is augmented by torque from the engine's turbine. At about 50 percent of operating speed the starter motor is shut off. The engine is now self-sustaining and accelerates itself to operating speed.
Typically, direct current (DC) motors are used as starter motors in such applications. However, as explained in the Wahl Patent, for a given power supply DC motor torque-versus-speed characteristics are fixed. Consequently, a DC starter motor must be sized to produce starting torque under the worst condition with the greatest engine drag, which is typically a cold soaked engine. Such DC starter motors are of a type commonly referred to as brush type motors, and have severe reliability problems due to the brush-commutator interface that can deteriorate rapidly due to high altitude starts. Further, the speed-torque profile of the motor is fixed and cannot be adjusted.
One solution to overcome the disadvantages associated with DC motors discussed in the Wahl Patent includes the use of the prime mover's AC generator as an AC starter motor. Generators, in combination with power converters, are commonly referred to as starter/generators (S/Gs). Furthermore, as pointed out in the Wahl Patent, the power converter can regulate the power consumption of the starter/generator from either a DC or AC source, therefore the converter can be controlled to provide any desired starting torque characteristic or torque-versus-speed profile. However, such systems are typically complex in their implementation, since main field flux must be provided at zero speed through the exciter machine and an associated inverter, and current in the main machine stator winding must be independently controlled via a second inverter to provide torque.
Accordingly, a need exists for a control system using an attached synchronous machine to accelerate a prime mover, such as a gas turbine engine without the additional complexities of extra control or power electronics for the AC excitation of the field winding of the exciter circuit during the engine start-up.